Ammonia values such as ammonia, ammonium carbonate and ammonium bicarbonate find many applications including, but not limited to, fertilizers and nutrients for metabolic processes. Numerous process streams contain ammonia values or nitrogen compounds that can yield ammonia values such as off gases from refining streams, syngas streams from steel manufacture, syngas from gasification of biomass, fermentation broths and solids, municipal and farm waste streams and the like. Advantageously these ammonia values are recovered for use where possible. A particularly attractive potential use for recovered ammonia values is in fermentation processes to supply nitrogen as a nutrient for the microorganisms.
For the recovery of ammonia values to be economically viable, the recovery processes should be cost effective and thus not only must the source of the ammonia values be available at a low cost, preferably as an existing disposal stream but also the capital and operating expenses for the recovery of the ammonia values must be relatively low. Additionally, the recovered ammonia values should be in a useable form. For instance, the product containing the ammonia values should not contain contaminants that would render the ammonia values unacceptable for their intended use. Where the product containing ammonia values is intended for use as a nitrogen nutrient for a fermentation process, the product should be substantially free of components deleterious to the microorganisms or cause an undesirable build-up of inert components in the fermentation process. Although the product containing ammonia values may be treated to remove such components, the additional unit operation adds to capital and operating costs.
Various flow schemes have been proposed to recover ammonia values from waste water that contains ammonium and ammonia. These processes generally involve raising the pH of the waste water to 9.0 or above, stripping the ammonia and then capturing it in an acidic scrubbing solution such as sulfuric acid (most common), hydrochloric acid or nitric acid. Conventional stripping towers, steam strippers, vacuum strippers and membrane systems (hollow fibers) have all been used at commercial and/or pilot scale.
Conventional methods for removal of ammonia, COS, and HCN from syngas prior to its use generally involves scrubbing with aqueous solutions to remove these compounds from the syngas with subsequent discharge of the scrubbing solutions to wastewater treatment or via alternate disposal methods.
Modern processes for ammonia removal include the water wash process in which the syngas is scrubbed by water, which dissolves the ammonia. The resulting scrubbing solution is pumped to an ammonia still where steam is used to strip out the ammonia. The ammonia vapors from the still can be processed to form ammonium sulfate, condensed to form a strong ammonia solution, incinerated or catalytically converted to nitrogen and hydrogen which are then recycled back into the gasifier.
Another process for ammonia removal from coke oven gas is the PHOSAM process developed by US Steel. This process absorbs the ammonia from the gas stream using a solution of monoammonium phosphate. The process produces saleable anhydrous ammonia, but operates at temperatures on the order of 50 degrees Celsius and pressures up to 190 psig (approximately 13 atmospheres of pressure gauge) in the stripper column. There is a need for a more robust and cost effective method for the treatment of syngas, particularly when used for biological transformation to useful liquid products such as ethanol, acetic acid or butanol.
Well known biological treatment processes used in concert with water based scrubbers can meet the objectives of high removal of ammonia, COS and HCN from syngas. Biological treatment processes can operate at atmospheric pressure and low temperatures without the excessive cost of expensive chemicals and operate without the generation of hazardous and/or toxic wastes. Biological treatment processing of ammonium, COS, and HCN absorbed into water from gas streams has been done before. Ammonia is, in general, removed using a slightly acidic or neutral pH scrubbing solution and this spent solution is sent to an aerobic wastewater treatment system where the ammonia is oxidized to nitrate and the nitrate subsequently reduced to nitrogen gas via denitrification, generally using an added organic electron donor such as methanol.
As generally described above ammonia may be removed from a system using a strong mineral acid such as hydrochloric acid (HCl) or sulfuric acid (H2SO4) to react with the alkaline ammonia, forming a solution containing an ammonium salt such as ammonium chloride (NH4Cl) or ammonium sulfate ((NH4)2SO4). As this method requires the input of a strong acid to the system, there is an added expense for the cost of the chemicals and also the increased design requirements of any vessels, piping, hoses, and other chemical handling equipment so that these components can withstand the acidic environment. The ammonium salt may be used or sold as a concentrated solution or may be processed and removed from the system.
Large amounts of ammonia containing materials can result from the utilization of biomass to produce biofuels. Biofuels production for use as liquid motor fuels or for blending with conventional gasoline or diesel motor fuels is increasing worldwide. Such biofuels include, for example, ethanol and n-butanol. One of the major drivers for biofuels is their derivation from renewable resources by fermentation and bioprocess technology. One available technology path to convert lignocellulosic biomass to ethanol is to convert lignocellulosic biomass to syngas (also known as synthesis gas, primarily a mix of CO, H2 and CO2 with other components such as CH4, N2, NH3, H25 and other trace gases) in a gasifier and then ferment this gas with anaerobic microorganisms to produce biofuels such as ethanol, propanol, n-butanol or chemicals such as acetic acid, propionic acid, butyric acid and the like. This technology path can convert all of the components to syngas with good efficiency (e.g., greater than 75%), and some strains of anaerobic microorganisms can convert syngas to ethanol, propanol, n-butanol or other chemicals with high (e.g., greater than 90% of theoretical) efficiency. Moreover, syngas can be made from many other carbonaceous feedstocks such as natural gas, reformed gas, peat, petroleum coke, coal, solid waste and land fill gas, making this a more universal technology path.
In the gasification of biomass, the preponderance of the nitrogen in the biomass is converted to ammonia. When the syngas is cooled and scrubbed to remove particulates and other contaminants, this ammonia is, to a large degree, removed in the scrubber/condensate flow stream. Treatment of this mass of ammonium requires a considerable sized waste water treatment system. If a significant fraction of this ammonium can be recovered for use in the fermentation itself and/or for export off site, a large savings in the capital and operating cost of waste water treatment can be realized, as little or no additional nitrogen needs to be purchased for the syngas fermentation and there may be the opportunity to market the remaining ammonium-nitrogen as a co-product.
These processes for the bioconversion of syngas to biofuels or biochemicals also provide a waste stream from the fermentation that contains microorganisms and other nitrogen containing compounds such as precipitated proteins. Recovery of ammonia values from this waste stream can be beneficial as the ammonia values may be recycled for use as a nitrogen source for supporting the fermentation or for other commercial value. In anaerobic digestion the biosolids in the waste stream are degraded and the cell nitrogen, proteins and other organic nitrogen-containing compounds converted to ammonia values. The higher the concentration of ammonia/ammonium produced, the higher the pH rises to because for each mole of ammonium formed a mole of alkalinity is concurrently formed. As the pH rises, significant amounts of non-ionized ammonia will be generated, in some cases high enough to cause inhibition of the anaerobic digestion process. In this situation additional water must be added to the digester to maintain the ammonia concentration below the threshold where the anaerobic digestion process is inhibited.
Methods are thus sought to recover ammonia values from waste streams, especially those generated in processes for making biofuels and biochemicals from syngas, which methods are economically attractive. Methods are sought for the anaerobic digestion of biosolids where the addition of water is not required to maintain the ammonia concentration below the threshold where the anaerobic digestion process is inhibited. Moreover, methods are sought that enhance the overall economics of processes for making biofuels and biochemicals from syngas.